Effects of electroplating variables on the composition and morphology of nickel/cobalt deposits plated through means of cyclic voltammetry

Transcription

1 Electrochimica Acta 47 (2002) 3447/ Effects of electroplating variables on the composition and morphology of nickel/cobalt deposits plated through means of cyclic voltammetry Allen Bai, Chi-Chang Hu *,1 Department of Chemical Engineering, National Chung Cheng University, Chia-Yi 621, Taiwan, ROC Received 12 March 2002 Abstract The effects of temperature and ph of the plating baths as well as the potential range and cycle number of cyclic voltammetry on the composition and morphology of Ni/Co deposits were systematically investigated. A reaction scheme corresponding to the Ni/ Co codeposition was proposed. Nickel /cobalt deposits with the composition approximately equal to their corresponding plating solutions (based on the metal content) were formed when ph of the plating bath was equal to 2.0 and the potential range of CV was between 0 and /1.2 V. This complete depression in the anomaly of Ni/Co codeposition was attributed to the anodic dissolution of the freshly deposited metal atoms resulting in the simultaneous dissolution of the adsorbed monohydroxide species occurring between /0.3 and 0 V. The effects of temperature and ph of the plating bath, the cycle number and potential range of CV on the morphology, loading, or crystalline structure of Ni/Co deposits were also compared. # 2002 Elsevier Science Ltd. All rights reserved. Keywords: Ni/Co deposits; Anomalous codeposition; Composition; Morphology; Cyclic voltammetry 1. Introduction Nickel, cobalt, and their alloys are important engineering materials in many applications because of their unique properties, such as magnetic [1,2], wear-resistant [3], heat-conductive [4,5], and electrocatalytic properties [6,7]. Moreover, Ni/Co oxides could be used in batteries [8,9]. Since the physicochemical properties of alloys are seriously affected by their compositions as well as structures [10 /16], a reliable control of the composition and structure for these engineering alloys is an important issue for their wide applications. The electroplating of Ni /Co alloys has been recognized as an anomalous codeposition [11,14 /17]. There are several researchers describing the above anomalous phenomena, e.g. the competition adsorption of metal hydroxides, monohydroxides, or metal ions since the * Corresponding author. Tel.: / x33411; fax: / address: chmhcc@ccu.edu.tw (C.-C. Hu). 1 ISE Member. hydrogen evolution reaction resulted in a local increase in ph near the deposit /electrolyte interface [15,18 /21]. A generally accepted mechanism/model proposed for this electroplating behavior due to the formation and adsorption of metal hydroxyl ions on the deposits can be simple expressed as following [15,22 /28]: 2H 2 O2e H 2 2OH (1) (side reaction on the cathode) M 2 OH M(OH) (2) M(OH) 0 M(OH) ads (3) (electrostatic force of the cathode) M(OH) ads 2e MOH (4) where M represents Co and Ni atoms. The newly formed OH in Eq. (4) favors the further formation of M(OH) and enhances the adsorption of M(OH). Since the adsorption ability of a less noble metal hydroxide (e.g. Co(OH) ) is considered to be higher than that of a more noble metal hydroxide (e.g. Ni(OH) ) [24/27], Co is enriched within the Ni/Co alloys. To circumvent this anomaly, Ni /Co alloys can /02/$ - see front matter # 2002 Elsevier Science Ltd. All rights reserved. PII: S ( 0 2 )

2 3448 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/3456 be normally deposited from molten salts [29]. Since the steps and procedures of a molten-salt process are relatively complicated, it is more desirable to search a new technique to reduce the anomaly of Co/Ni plating in aqueous media. In our preliminary study [30], the anomalous codeposition of Ni /Co alloys was inhibited using cyclic voltammetry in a suitable potential range. This interesting finding was attributable to the damage of the absorbed monohydroxide species (i.e. M(OH) ) by means of anodic dissolution. In this work, the effects of temperature and ph of the plating baths as well as the cycle number and potential range of CV on the composition, loading/thickness and morphology of Co /Ni deposits were compared with support the above inference. In addition, a reasonable reaction scheme responsible for the above interesting phenomena during the codeposition of Ni /Co alloys by using cyclic voltammetry was proposed and discussed. 2. Experimental details The Ni /Co deposits were electroplated onto commercial pure (99.5%) 1 /2 cm Cu plates. These Cu plates were first cleaned with trichloroethylene, rinsed with pure water, and then, anodized at 40 ma cm 2 in a 0.1 M NaOH solution for 10 min. After anodizing, these plates were cathodically polarized at 40 ma cm 2 in another 0.1 M NaOH solution for 1 min, vibrated in an ultrasonic bath for 5 min, acid-cleaned with 0.1 M HCl for 2 min, and finally rinsed with pure water. The exposed geometric area of these pretreated Cu substrates is equal to 1 cm 2 while the other surface areas were insulated with PTFE (polyterafluorene ethylene) films before the deposition. The composition of NiCl 2 / 6H 2 O:CoCl 2 /6H 2 O in the deposition baths was specified as 8:2, 6:4, 4:6, and 2:8. In addition, pure nickel and cobalt deposits were also prepared in this work for comparison purposes. The total concentration of metal chlorides was kept constant (i.e. 0.3 M) in the plating bath with ph of 2.0 and 3.0 (adjusted with 1 M HCl). The Cu substrates were coated with Ni/Co deposits by means of cyclic voltammetry in two potential ranges (i.e. 0to/1.2 V and /0.35 to /1.2 V). In order to study the effects of cycle number on the properties of alloys, the electroplating was interrupted when 10, 20 or 30 cycles of CV were performed. Cyclic voltammetry for the electroplating of Ni /Co deposits was performed by mean of an electrochemical analyzer system, CHI 633A (CH Instruments, USA). The scan rate of CV was kept at 20 mv s 1. All experiments were carried out in a three-compartment cell. An Ag/AgCl electrode (Argenthal, 3 M KCl, V vs. SHE at 25 8C) was used as the reference and a platinum mesh with an exposed area of 4 cm 2 was employed as the counter electrode. A Luggin capillary, whose tip was set at a distance of 2 mm from the surface of the working electrode, was used to minimize error due to IR drop in the electrolytes. The average composition of all deposits was measured by means of an energy-dispersive X-ray (EDX) spectroscope with standards at three points using a scanning electron microscope (SEM, JEOL JSM35). The mean error of this EDX analysis is about 9/1.5 atomic percents (at.%). Morphologies of these deposits were examined at 2000 and 6000 times using the same SEM. The loading of all alloys is the weight difference of the electrode without the PTFE coating before and after the application of CV plating through a microbalance with an accuracy of 20 mg (Sartorius BP 211D, Germany). All solutions used in this work were prepared with reagent water (MILLI-Q SP, Japan) at 18 MV cm. All reagents not specified in this work are Merck, G.R. Solution temperature during the electroplating was maintained at 25, 38 or 50 8C with an accuracy of 0.1 8C by means of a water thermostat (HAAKE DC3 and K20). 3. Results and discussion 3.1. Effects of electroplating variables on the composition of Ni/Co alloys Typical cyclic voltammograms (e.g. the fourth cycle) for the CV plating of Ni, Co and Ni/Co deposits in the baths with ph of 2.0 and 3.0 are shown as curves 1/3 in Fig. 1a, b, respectively. In Fig. 1a, three reaction regions corresponding to the cathodic deposition, no reaction and anodic oxidation processes are clearly found on all CV curves. Note that at potentials negative to /0.7 V on both positive and negative sweeps of all curves, cathodic currents are clearly found, indicating that nickel and cobalt ions can be deposited onto the Cu substrate in the cathodic deposition region. However, neither cathodic deposition nor anodic oxidation (e.g. dissolution) occurs when electrode potentials are located between /0.35 and /0.7 V. Moreover, the larger anodic currents on curve 2 (i.e. the pure cobalt deposit) at potentials positive than /0.35 V indicate the more oxidative property of Co metals. On the other hand, several features have to be considered when ph of the baths is equal to 3 (see Fig. 1b). First, the onset potential of the anodic oxidation region for both pure Co and binary Ni /Co deposits was shifted from /0.35 to /0.5 V. Second, the onset potential of cathodic deposition occurs at approximately /0.6 V for these two deposits. Third, the pure nickel deposit is completely passive as potentials positive than approximately /0.9 V and the onset potential of electroplating is negatively shifted from /0.7 to /0.9 V. Fourth, cathodic currents

3 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/ Fig. 2. Cyclic voltammograms of Ni/Co deposits for the (1) first; (2) 15th; and (3) 30th cycle measured at 20 mv s 1 from the baths containing 0.18 M NiCl 2 /6H 2 O and 0.12 M CoCl 2 /6H 2 O with ph of 2.0 between /0.35 and /1.2 V. Fig. 1. Cyclic voltammograms of Ni/Co deposits measured from the baths of (1, */) NiCl 2 /6H 2 O; (2, */ */) CoCl 2 /6H 2 O; and (3, ---) NiCl 2 /6H 2 O:CoCl 2 /6H 2 O/6:4 between 0 and /1.2 V with ph of (a) 2.0 and (b) 3.0 at 20 mv s 1. attributable to the Ni /Co codeposition (see curve 3) are much higher than that of Co deposition (see curve 2), which is not found in the plating bath with ph of 2.0. In this work, there is an obvious change in i /E responses during the first three cycles of CV (probably due to the nucleation of alloys and the incomplete coverage of the substrate surface) in plating the Ni /Co alloys. On the other hand, i /E curves show the same behavior (i.e. similar in shape and currents) in the whole potential region of investigation in the following cycles, indicating the typical voltammetric behavior of potential-cycling deposition. However, when the cyclic voltammograms do not include the anodic loop, the cathodic currents of alloy deposition are gradually decreased with the cycle number (see Fig. 2). The above results and discussion indicate the existence of interactions between Co (and Ni) ions and OH, which should affect the codepositing mechanism of Ni /Co alloys [25]. On the basis of the above results and discussion, effects of the potential range of CV, ph and temperature of the plating baths on the composition of Ni /Co deposits electroplated by cyclic voltammetry from the baths with different compositions are systematically compared and the results are summarized in the Table 1. In this table, the first column is the Co content in the plating bath while the corresponding Co contents within all deposits plated in different conditions are shown from the second to the eleventh columns. A comparison of the data in columns 1 and 2 reveals that the difference in the composition between baths and deposits is always less than 5 at.%. Thus, the anomaly of Ni/Co codeposition is effectively inhibited by the employment of cyclic voltammetry with the potential range between 0 and / 1.2 V when ph of baths is equal to 2.0. From a comparison of the data in columns 2/4, the composition of Ni /Co deposits is not influenced by varying the temperature of plating baths from 25 to 38 or 50 8C. Thus, the effect of plating temperature on the composition of Ni/Co deposits is negligible. On the other hand, from a comparison of the data shown in columns 2 (i.e. potential range of 0 to /1.2 V) and 5 (i.e. potential range of /0.35 to /1.2 V), an obvious difference in compositions between the binary deposits and their corresponding baths is clearly found for the latter case. The above results reveal that the anodic process at potentials positive than /0.35 V on the positive and negative sweeps of deposition CVs in Fig. 1 can significantly depress the anomalous behavior of Ni / Co codeposition. This statement is also supported by a comparison of the data shown in columns (3, 6) and (4, 7) (i.e. plating temperature of 38 and 50 8C). Therefore, the anodic process can presumably be attributed to the anodic dissolution of surface metal atoms, resulting in the simultaneous dissolution of newly adsorbed monohydroxide ions formed during the cathodic codeposition of Ni/Co alloys. Effects of ph on the composition of Ni/Co deposits can be obviously found from a comparison of the data shown in columns 2/7 and8/11 in Table 1. Note the data shown in columns 2 (ph 2.0) and 8 (ph 3.0) that

4 3450 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/3456 Table 1 Effects of the potential range of CV, temperature and ph of plating baths on the composition of Ni Co deposits electroplated by cyclic voltammetry for 30 cycles at 20 mv s 1 Co/(CoNi) in bath (%) ph 2.0 ph 3.0 0to1.2 V 0.35 to 1.2 V 0 to 1.2 V 0.35 to 1.2 V 25 8C 38 8C 50 8C 25 8C 38 8C 50 8C 25 8C 50 8C 25 8C 50 8C the anomalous phenomenon is clearly found for the deposits plated from the baths of ph 3.0 although the potential range of CV deposition for both cases is between 0 and /1.2 V and the plating temperature is equal to 25 8C. On the other hand, a comparison of the data shown in columns 5 /7 reveals the insignificant influence of the plating temperature on the composition of Ni/Co deposits, which has been found in the case with the anodic loop. Moreover, this phenomenon is also found in the case of ph 3.0, revealing the negligible effect of plating temperature on the composition of Ni / Co deposits. Note that a comparison of the data shown in columns 8 and 10 (i.e. ph 3.0 and plating temperature of 25 8C), the anomalous degree of Ni/Co deposits is increased when the upper potential limit of CV (E SU )is changed from 0 (column 8) to /0.35 V (column 10). Accordingly, the anodic process is also able to depress the anomaly of Ni/Co electroplating in a less acidic solution. From all the above results and discussion, ph of the plating baths and the potential range of CV during the electrodeposition are concluded to be the key factors influencing the anomaly of Ni /Co electroplating. Effects of cycle number and potential range of CV on the loading of deposits and the current efficiency of electroplating are summarized in Table 2. The current efficiency of CV deposition is defined as the following equations: Dw 2F=mMW C:E: 100% (5) X n i1 (½q c ½ q a ) mmw x58:71(1x)58:93 (6) where Dw, q c, q a, mmw, and x indicate the loading of deposits, the voltammetric charges of cathodic deposition and anodic oxidation on every sweep, the mean molecular weight of Ni /Co alloys, and the molar fraction of nickel in the deposits. Note the data in columns 2/4 that the deposits were plated by using CV between 0 and /1.2 V in a bath containing NiCl 2 / 6H 2 O:CoCl 2 /6H 2 O/8:2 with ph of 2.0 at 25 8C. A comparison of these data reveals that the metal composition within the Ni /Co deposits is not influenced by changing the cycle number of deposition since the difference in composition between the plating baths and deposits is always less than 5 at.%. On the other hand, the loading of Ni /Co deposits is proportional to the cycle number of CV, which has been confirmed by their thickness (see Section 3 for Fig. 4). This result indicates the continuous electroplating of Ni /Co deposits with increasing the number of CV cycles. From a Table 2 Effects of the CV cycle on the loading (Dw), current efficiency (CE) and composition of Ni Co deposits electroplated from the solution containing 0.06 M CoCl 2 6H 2 O and 0.24 M NiCl 2 6H 2 Oat20mVs 1 Co/(CoNi)20% in bath b 0to1.2 V 0.35 to 1.2 V Ten cycles Twenty cycles Thirty cycles Ten cycles Twenty cycles Thirty cycles Dw (mg) CE (%) Co/(CoNi) (%) Ni/(CoNi) (%) O (at.%) Cu (at.%) a 0 a 0 a 0 a 0 a a This element was not found. b ph of the baths is equal to 2.0.

5 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/ comparison of the data in columns 2 and 5, the loading is heavier for the deposit without the anodic loop (i.e. potential range of /0.35 to /1.2 V), revealing a partial dissolution of the freshly plated deposit in every anodic loop. This is further supported by a comparison of the data in columns 3 and 6 or columns 4 and 7. Moreover a comparison of the oxygen content in columns 4 and 7 (or columns 2 and 5 as well as columns 3 and 6) reveals that the incorporation of oxygen in the deposits is enhanced by changing the potential range from 0 to /1.2 to /0.35 to /1.2 V. Accordingly, a larger amount of hydroxyl species should be adsorbed at the deposit / solution interface for the case without the anodic loop. Similar phenomena are also obviously found for the deposits prepared from the solutions with ph of 3.0 (not shown here), further supporting the above statement. Based on the above results and discussion, the proposal that the anodic process is a dissolution reaction of the surface metal atoms, resulting in a simultaneous dissolution of the newly adsorbed monohydroxide ions and inhibiting the anomaly of Ni/Co codeposition is reasonable and acceptable. On the basis of the above results and discussion, a reasonable reaction scheme describing the Ni /Co codeposition by using cyclic voltammetry is proposed and shown in Fig. 3. In general, Ni 2 and Co 2 (in the freeion form) can be normally plated onto the substrate (i.e. through path (a)). On the other hand, hydrogen evolution (a main side reaction for the cathodic deposition) should cause a local increase in ph near the electrode surface (i.e. reaction 1) because obvious hydrogen bubbles were found to adsorb on the surface of Ni/ Co deposits when potentials were swept between /1.1 and /1.2 V. Therefore, Co(OH) and Ni(OH) should be formed (i.e. reaction 2) at the deposit-solution interface during hydrogen evolution. These newly formed monohydroxide ions will be adsorbed on the surface of Ni /Co deposits (i.e. reaction 3) and increase the influence of path (b) on the Ni/Co codeposition when a DC electroplating is employed. Since Co is enriched within the binary Ni /Co deposits, the anomalous phenomenon of nickel /cobalt electroplating can be attributed to the stronger adsorption of Co(OH) [3 /5,8]. From the data in Table 1, the anomalous Fig. 3. A reaction scheme of the Ni/Co electroplating by means of cyclic voltammetry. codeposition is obvious when the potential range of CV does not include the anodic dissolution process. This situation is similar to the DC electroplating case since the adsorbed M(OH) species are not effectively damaged. On the other hand, the anodic process on the CV curves does depress the anomalous phenomenon of Ni/Co codeposition, especially in a more acidic baths (e.g. ph 2.0). Thus, on the basis of loading data shown in Table 2, this anodic process is expected to be the dissolution of the freshly deposited metal atoms, resulting in the simultaneous dissolution of M(OH) ads. Under such situations, path (a) (i.e. the normal electroplating of Ni 2 and Co 2 onto the substrate) becomes the predominant process in the following cathodic deposition region, promoting the content of Ni within the deposits. Since this inhibition of Co enrichment is still found when ph of the plating baths is equal to 3.0, the freshly deposited metal atoms are expected to partially dissolve during the anodic process at potentials positive than /0.5 V (see Fig. 1b). However, the anomaly of Ni/Co codeposition is obvious when ph of the plating baths is equal to 3.0, indicating that dissolution of the freshly deposited metal atoms is depressed in a less acidic medium, inhibiting the complete dissolution of adsorbed M(OH) species. Moreover, the anodic dissolution of metals may be partially inhibited by the formation of M(OH) 2 in this less acidic bath since a higher concentration of OH also favors the formation of hydroxides (i.e. reaction 4). If this is the case, anomaly of the Ni /Co electroplating should be found when the ph of the plating baths is changed from 2.0 to 3.0. From the above results and discussion, the ph of the plating bath is believed to have two effects on the Ni /Co codeposition. First, an increase in proton concentrations should decrease the formation of M(OH), resulting in an increase in the concentration of free metal ions, depressing the contribution of path (b). Second, an increase in proton concentrations also favors the dissolution of metal atoms during the anodic loop, inhibiting the formation of M(OH) 2 and/or M(OH) ads. When these effects predominate in the Ni /Co codeposition, the already-deposited surface metal atoms will be oxidized to form M 2, rendering the dissolution of interfacial M(OH) ads in the anodic loop and inhibiting the anomalous codeposition. In fact, this reaction scheme is also applicable for the cathodic deposition of Ni(OH) 2 [19,31], which have been employed to prepare nickel hydroxides for the application of batteries and electrochromic devices. Note that the hydrogen evolution reaction causes an increase in OH concentrations, favoring the formation and adsorption of M(OH). These M(OH) ads species will be further coordinated with OH to form M(OH) 2 on the substrate surface when ph of the plating baths becomes higher. Moreover, since the water is decomposed into adsorbed hydrogen atoms and OH ions near the

6 3452 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/3456 electrode surface during hydrogen evolution, these OH ions should easily combine with M(OH) to form M(OH) 2 and adsorbed on the substrate. Therefore, most M(OH) ads will be in the form of M(OH) 2 ads, resulting in the cathodic deposition of M(OH) 2 when the rate of hydrogen evolution is faster than the kinetics of metal deposition in the cathodic plating region Effects of electroplating variables on the morphologies of Ni /Co alloys The cross-sectional profile of a Ni/Co deposit with 30 cycles of CV is shown in Fig. 4. This deposit was prepared from a solution containing 0.06 M CoCl 2 / 6H 2 O and 0.24 M NiCl 2 /6H 2 O at ph of 2.0 and temperature of 25 8C with the potential range of CV between 0 and /1.2 V. The mean thickness of this Ni/ Co deposit is about 3.78 mm, which is found to be proportional to the cycle number of CV deposition since the thickness of deposits with ten and 20 cycles is equal to 0.65 and 1.52 mm, respectively. From a comparison of the loading and thickness data, the thickness of Ni /Co deposits is directly proportional to their loading. Effects of the potential range of CV on the morphologies of Ni/Co deposits with different compositions can be illustrated from a comparison of their SEM photographs. Typical results plated between 0 and /1.2 V (a/ c) and between /0.35 and /1.2 V (d/f) are show in Fig. 5. The thickness of the deposits shown in Fig. 5a/c is about half of those shown in Fig. 5d/f because the anodic dissolution loop is included in the deposition CV for the former three deposits. In fact, the loading/ thickness only exhibits an effect on the grain size while their grain shape is not affected by the loading/thickness of deposits (see below). From an examination of Fig. 5, Fig. 4. The cross-sectional profile of a Ni/Co deposit with 30 CV cycles from the baths contain 0.24 M NiCl 2 /6H 2 O and 0.06 M CoCl 2 / 6H 2 Oat20mVs 1. the morphology of these deposits is significantly affected by varying the potential range of CV. For instance, the pure Co deposit plated from 0 to /1.2 V shows starfruit-like grains with a relatively compact nature (see Fig. 5a) while spherical grains with a very rough nature are found when this deposit is electroplated between / 0.35 and /1.2 V (see Fig. 5d). In addition, for all deposits plated by the CVs without the anodic loop, the grain boundaries are granular and obvious (see Fig. 5d/ f). From a comparison of Fig. 5c and f, the relatively smooth morphology of the former deposit indicates the significant dissolution of surface nickel atoms. Similar phenomena are also found from a comparison of Fig. 5b and e (i.e. Ni 6 Co 4 ), further supporting the proposal that the anodic oxidation process is attributed to the dissolution of surface metal atoms. Thus, the inhibition of anomalous codeposition of Ni /Co alloys is attributed to this dissolution reaction damaging the M(OH) ads species. Note that the morphology of Ni /Co deposits is strongly dependent upon the composition while the XRD patterns indicate that the crystalline structure of Ni /Co alloys with different compositions is similar. From the above results and discussion, the reaction scheme of Ni/Co plating in the acidic media proposed in Fig. 3 is supported by the dissolving morphology of Ni /Co deposits. In addition, the reaction path for the M(OH) 2 and/or M(OH) ads formation in a weakly acidic medium (i.e. reactions 4 and 5) is further confirmed from a comparison of the oxygen content within the Ni/ Co deposits prepared from the plating baths with ph of 2.0 and 3.0, respectively. Note that the oxygen content of Ni/Co deposits prepared by CV with the anodic loop from the baths of ph 2.0 is ranged from approximately 3 to 5 at.% while 7/10 at.% of oxygen is found on the deposits prepared from the baths of ph 3.0. Moreover, the oxygen content of Ni/Co deposits prepared by CV without the anodic loop from the baths of ph 2.0 is confirmed to be between 7 and 11 at.% meanwhile the deposits prepared from the baths of ph 3.0 is between 11 and 15 at.%. The above results indicate that in the plating bath with a low ph value, the combination of metal ions (M 2 ) and OH to form the adsorbed M(OH) 2 and/or M(OH) is depressed. Thus, almost all metal atoms should be dissolved as the free-ion form in the plating bath during the anodic dissolution process. When ph of the plating bath is changed to be higher, however, the formation of adsorbed M(OH) 2 and/or M(OH) becomes relatively easier. Hence, the surface of Ni /Co deposits is probably adsorbed (partially) by M(OH) 2 and/or M(OH). Therefore, certain adsorbed hydroxyl species should be incorporated into the alloys, which favor the anomaly of Ni /Co codeposition. This incorporation of oxygen into the Ni /Co matrix results in the increase in oxygen content within the Ni /Co deposits prepared from a less acidic bath.

7 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/ Fig. 5. SEM photographs of Ni/Co deposits plated at 20 mv s 1 for 30 cycles (a /c) between 0 and /1.2 V and (d/f) between /0.35 and /1.2 V from (a, d) CoCl 2 /6H 2 O; (b, e) NiCl 2 /6H 2 O:CoCl 2 /6H 2 O/6:4; and (c, f) NiCl 2 /6H 2 O baths with the bath temperature of 50 8C and ph of 2.0. Effects of the plating temperature on the morphology of Ni /Co deposit can be illustrated by the comparison of Figs. 5 and 6. Note that the morphology of Ni prepared at a lower temperature (i.e. Fig. 6c) is very similar to that in Fig. 5f. In addition, the Ni/Co deposit prepared at a lower temperature (i.e. Fig. 6b) has the morphology between Fig. 5b and e. A similar phenomenon is also found for the pure Co deposit. These results suggest that the dissolution rate of the freshly deposited atoms is slower at a lower temperature although the rate of dissolution is rapid enough to completely dissolve the adsorbed M(OH) species. Since a higher temperature of the plating baths should favor the diffusion of metal ions, the deposition rate should be promoted by the increase in plating temperatures. However, the electron transfer rates of Ni /Co dissolution in the anodic loop should be also enhanced by the increase in the plating temperature. Accordingly, the anomaly of Ni /Co codeposition is approximately independent of the plating temperature, which has been found in Table 1. The effects of ph in the plating baths on the morphology of Ni /Co deposits are illustrated from a comparison of Figs. 6 and 7. First, the morphologies of Ni, Ni /Co and Co deposits are significantly influenced by ph of the deposition solutions. This effect is not only attributed to the composition difference between the binary Ni /Co deposits but also attributed to the intrinsic effect of ph on the textural properties (e.g. oxygen content, crystalline structure and/or preferred orientation, etc.) of all deposits since the morphology of pure Ni and pure Co deposits is also influenced by ph of the plating baths. In fact, the effects of ph on the textural properties of Watts nickel deposits have been discussed previously and led to a similar conclusion [32], supporting the above statements. Second, the influences of ph on the morphology of Ni, Ni/Co and Co deposits are different from each other. These differences may be attributed to the different oxidation natures of Ni, Ni / Co and Co deposits in the less acidic media, which have been found on their deposition CVs (see Section 3 for Fig. 1b). On the other hand, since the anodic process of CV has been revealed to depress the anomaly of Ni/Co codeposition while the anomalous phenomenon is clearly observed when the deposits are prepared in the

8 3454 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/3456 Fig. 6. SEM morphologies of Ni/Co deposits plated at 20 mv s 1 for 30 cycles between 0 and /1.2 V from (a) CoCl 2 /6H 2 O; (b) NiCl 2 / 6H 2 O:CoCl 2 /6H 2 O/6:4; and (c) NiCl 2 /6H 2 O baths with the bath temperature of 25 8C and ph of 2.0. less acidic solutions (see Table 1), the adsorbed M(OH) species are thus considered to partially dissolve in the anodic loop. Hence, the combined contributions of paths (a) and (b) (see Fig. 3) during the electroplating of these deposits is expected to influence the morphologies of these deposits, which may also affect the other textural properties. The increase in loading (or thickness) of Ni /Co deposits resulting in the more obvious grain boundaries and the larger grain size can be examined by the morphologies of Ni /Co deposits with different plating cycles of CV. Typical results for various Ni/Co deposits plated at 20 mv s 1 for 10 and 20 cycles between 0 and /1.2 V are shown in Fig. 8. From an examination of Fig. 5a/c and Fig. 8, larger grains are clearly found on the deposits with a larger cycle number of the plating CV. However, their grain shape seems not to be affected by the increase in cycle number for any deposits. Accordingly, the mechanism of Ni /Co codeposition is presumably independent of the applied cycles of CV. Fig. 7. SEM morphologies of Ni/Co deposits plated at 20 mv s 1 for 30 cycles between 0 and /1.2 V from (a) CoCl 2 /6H 2 O; (b) NiCl 2 / 6H 2 O:CoCl 2 /6H 2 O/6:4; and (c) NiCl 2 /6H 2 O baths with the bath temperature of 25 8C and ph of 3.0. The above results support the proposal that the grain size of Ni /Co deposits is directly proportional to their loading. Similar results are also found for the deposits prepared at a higher temperature with the deposition CV without the anodic loop (not shown here), indicating that the electroplating rate is significantly promoted by increasing the bath temperature. From all the above results and discussions, three major effects influencing the morphology of Ni /Co deposits plated by CV are proposed in the following. First, the morphology of a Ni/Co deposit is changed with its composition and the potential range of CV deposition. Second, the dissolution in the anodic loop decreases the sharpness and/or the size of grains. This effect can be clearly found for the case deposited in a strongly acidic bath (e.g. Fig. 5). When ph of the plating becomes less acidic, the rate of metal dissolution is slower and the oxygen content of Ni /Co deposits is increased. Third, the higher loading due to a larger number of deposition cycles or a higher plating temperature results in the formation of larger grains while

9 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/ Fig. 8. SEM morphologies of Ni/Co deposits plated at 20 mv s 1 for (a/c) ten and (d /f) 20 cycles between 0 and /1.2 V from (a, d) CoCl 2 /6H 2 O; (b, e) NiCl 2 /6H 2 O:CoCl 2 /6H 2 O/6:4; and (c, f) NiCl 2 /6H 2 O baths with the bath temperature of 50 8C and ph of 2.0. the deposition mechanism seems to be not influenced by changing the temperature and cycle number of CV deposition XRD patterns of Ni /Co alloys The effect of cycle number and potential range of CV on the crystalline structure of Ni /Co deposits is examined by the XRD patterns. Typical results for the Ni /Co deposits plated from the bath containing 0.24 M NiCl 2 /6H 2 O and 0.06 M CoCl 2 /6H 2 OwithpHof2.0 and temperature of 25 8C for ten, 20 and 30 cycles of deposition CVs between 0 and /1.2 V are shown as curves a/c in Fig. 9, respectively. In addition, the deposit plated from the same solution with the potential range of CV between /0.35 and /0.12 V for 30 cycles is also shown as curve dinfig. 9. Note on curves a/c that the intensity of diffraction peaks corresponding to the Cu substrate is gradually decreased with the cycle number of CV, indicating an increase in thickness of the deposits. On the other hand, small diffraction peaks corresponding to Ni /Co alloys are found on these three curves, indicating the microcrystalline structure of these deposits since the SEM photographs show the obvious grain boundary of these deposits. From a comparison of curves c and d, the intensity of diffraction peaks for the deposit plated by CV without the anodic dissolution process (i.e. Fig. 9d) is lower than that with the anodic loop (Fig. 9c) although the grain boundary for the former deposit is more obvious than that for the latter deposit (e.g. see Fig. 5). Moreover, the latter deposit (ca. 4 mm) is thinner than the former deposit (ca. 6 mm). These results reveal that the deposit plated by CV without the anodic loop is concluded to be microcrystalline. This is attributable to the relatively high content of oxygen incorporated within the Ni /Co alloy matrix. 4. Conclusions The anomaly of Ni /Co electroplating was completely inhibited by means of cyclic voltammetry between 0 and

10 3456 A. Bai, C.-C. Hu / Electrochimica Acta 47 (2002) 3447/3456 References Fig. 9. XRD patterns of Ni/Co deposits plated at 20 mv s 1 for (a) ten; (b) 20; (c) 30 cycles between 0 and /1.2 V and (d) for 30 cycles between /0.35 and /1.2 V from the bath contain 0.24 M NiCl 2 /6H 2 O and 0.06 M CoCl 2 /6H 2 O. /1.2 V when the deposit was plated from the bath with ph of 2.0. The freshly deposited metals were partially dissolved between /0.35 and 0 V in the anodic loop, especially in a more acidic bath, depressing the formation of adsorbed M(OH) species and favoring the path of normal codeposition of free metal ions. A higher ph bath, however, favored the formation of M(OH) 2 / M(OH) and depressed the dissolution of alreadydeposited metals, increasing the anomaly of deposits. Morphologies of Ni /Co deposits are strongly dependent on the composition, the potential range of CV and the ph of plating baths. The grain size of Ni /Co deposits increased with increasing the cycle number of CV deposition or the plating temperature because of a larger loading of the deposits. The grain boundaries became unobvious when the dissolution reaction in the anodic loop exhibited a significant contribution on the Ni /Co electroplating. Acknowledgements The financial support of this work, by the National Science Council of the Republic of China under contract no. NSC E , is gratefully acknowledged. [1] US Patent 3,274,079 (1966), to F. Passal; Chem. Abstr. 66 (1967) 16032k. [2] East German Patent 52,869 (1966), to F. Friemel; Chem. Abstr. 67 (1967) 17376h. [3] Jpn. Patent 74, (1974), to M. Jato; Chem. Abstr. 82 (1975) 91774n. [4] Jpn. Patent 75,02377 (1975), to S. Ichioka, T. Koda; Chem. Abstr. 83 (1975) 87387s. [5] V.B. Singh, V.N. Singh, Plating. Surf. Finish. 63 (7) (1976) 34. [6] C.-C. Hu, C.-Y. Weng, J. Appl. Electrochem. 30 (2000) 499. [7] C.-C. Hu, A. Bai, J. Appl. Electrochem. 31 (2001) 565. [8] F. Maurel, B. Knosp, M. Backhaus-Ricoult, J. Electrochem. Soc. 147 (2000) 78. [9] C.C. Wang, K.S. Goto, S.A. Akbar, J. Electrochem. Soc. 138 (1991) [10] W. Schwarzacher, D.S. Lashmore, IEEE Trans. Hagn. MAG-32 (1996) [11] S.S. Djokic, M.D. Maksimovic, in: J. O M Bockris (Ed.), Modern Aspects of Electrochemistry, No. 22, Plenum Press, New York, 1992, p [12] T. Osaka, Electrochem. Acta 44 (1999) [13] T. Osaka, M. Takai, K. Ohashi, M. Saito, K. Yamada, Nature 392 (1998) 796. [14] E.M. Kakuno, D.H. Mosca, I. Mazzaro, N. Mattoso, W.H. Schreiner, M.A.B. Gomes, M.P. Cantao, J. Electrochem. Soc. 144 (1997) [15] K.-M. Yin, J. Electrochem. Soc. 144 (1997) [16] E. Beltowska-Lenman, A. Riesenkampf, Surf. Technol. 11 (1980) 349. [17] Y.-P. Lin, J.R. Selman, J. Electrochem. Soc. 140 (1993) [18] A. Brenner, Electrodeposition of Alloys, vol. 1 /2, Academic Press, New York, [19] H. Dahms, I.M. Croll, J. Electrochem. Soc. 112 (1965) 771. [20] S. Biallozor, M. Lieder, Surf. Technol. 21 (1984) 1. [21] M. Motlosz, J. Electrochem. Soc. 140 (1993) [22] M. Ramasubramanian, S.N. Popova, B.N. Popov, R.E. White, K.-M. Yin, J. Electrochem. Soc. 143 (1996) [23] S. Hessami, C.W. Tobias, J. Electrochem. Soc. 136 (1989) [24] K.-M. Yin, J.-H. Wei, J.-R. Fu, B.N. Popov, S.N. Popova, R.E. White, J. Appl. Electrochem. 25 (1995) 543. [25] J. Vaes, J. Fransaer, J.-P. Celis, J. Electrochem. Soc. 147 (2000) [26] K.-M. Yin, S.-L. Jan, Surf. Coat. Technol. 79 (1996) 252. [27] K.-M. Yin, C.-C. Lee, J. Chem. Tech. Biotechnol. 70 (1997) 337. [28] L.T. Romankiw, in: L.T. Romankiw (Ed.), Electrodepoistion Technology, Theory and Practice, PV 87-17, The Electrochemical Society Proceedings Series, Pennington, NJ, 1987, p [29] P.-Y. Chen, I.-W. Sun, Electrochim. Acta 46 (2001) [30] A. Bai, C.-C. Hu, in: Meeting Abstract of the 2001 Joint International Meeting, No. 657, San Francisco, CA, September 2/7, [31] Y. Sasaki, T. Yamashita, Thin Solid Films 334 (1998) 117. [32] C.-C. Hu, C.-Y. Lin, T.-C. Wen, Mater. Chem. Phys. 44 (1996) 233.